KOYAMA Etsutaro1 NIKKUNI Masaaki1 ODOHIRA Tetsu1
For a long time now, we have appreciated the standard 4-20 mA signal for 1to-1 analog communication lines in various plants. The time has now come for a new type of communication, namely, fieldbus, which features digital transmission on 1-to-1 communication lines. We have just developed an EJA series differential pressure transmitter with FOUNDATION™ fieldbus communication to be used as one of the main sensors in a plant. The fieldbus communication is to be supported by all models of the EJA series. That is, all new EJA models are equipped with the Standard Resource Block and the Standard Analog Input Block.
This paper describes the structure and the characteristics of the EJA transmitter with fieldbus communication.
|Figure 1 External View
of the EJA Series
Differential pressure transmitters, which serve as the core of field instrumentation, are widely used to measure the flow rate, pressure and density of liquids, gases and steam, as well as the level of liquid in a tank.
There is strong demand from industrial plants for more accurate plant operation and labor savings in both plant operation and management as a means of improving yields. Not only are field devices required to be more precise and stable, but they must also be more intelligent in order to cut the cost of labor for plant operation and maintenance. In 1988, Yokogawa Electric released their first pressure transmitter with the BRAIN communication function-a communication protocol in which the 4-20 mA analog signal is mixed with digital signals. Since then, they have continued to release newer models of these pressure transmitters as well as striving to produce increasingly intelligent pressure transmitters, such as the DPharp series.
Recently, the focus has turned to the standard fieldbus (with its multidrop connection and full digital transfer) that will replace the conventional BRAIN communication with 1-to-1 connection. This paper introduces the EJA series pressure transmitter that supports the fieldbus protocol advocated by the Fieldbus Foundation. Figure 1 is an external view of the transmitter.
We now describe the design concepts and features of the fieldbus-enabled EJA series pressure transmitter.
|Figure 2 Components of the Transmitter|
Figure 2 shows the components of the differential pressure transmitter. The transmitter can be roughly divided into the capsule and the transmission block. The capsule comprises the pressure sensing unit, cover flange and process connector. The transmission block comprises the converter and terminal box and can be commonly used for all models in the EJA series. In addition, the transmitter can contain an optional LCD display for on-site monitoring.
1. Circuit Configuration
Figure 3 is a circuit diagram of a fieldbus-enabled EJA series transmitter.
Figure 3 Circuit Configuration
The circuit is divided into two blocks: an excitation circuit block that excites the resonant sensor to detect its eigenfrequency and a converter block that applies a computational process to signals in order to convert them into signals for output to the fieldbus. The converter block measures the signal frequency using a dedicated microprocessor and the time interval using a reference clock, so as to improve the resolution. This measurement is carried out continuously to further improve the resolution by means of averaging. Corrective computing is carried out so that a precise reading of differential pressure can be obtained.
The transmission block incorporates Yokogawa's proprietary MAU device (ASIC for fieldbus applications) and a communication controller for implementing fieldbus communication.
Figure 4 Functional Block Diagram
Figure 4 shows a functional block diagram of the transmitter. The functions of the fieldbus-enabled EJA transmitter model are classified into the following blocks:
Each block is described below.
In this paper, we discussed the features, structure and functionality of the fieldbus-enabled transmitter of the EJA series. This recently developed model is provided with the common functions of the fieldbus standard and prioritizes the need for an easy transition from the conventional instrumentation method. We are confident that this product will help spread fieldbus technology and enable future plants to achieve labor savings and precision operation.
As technical infrastructures such as engineering tools improve and fieldbus technology spreads, we believe that more advanced functions, which require only simple work operations, will be increasingly possible. We continue to monitor market trends and strive to offer solutions that meet an even wider variety of user demands.